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;a5Z7z0Z7 ! ` r � A rT F r Kll N0 '-0N`)U1 7ANCY APR2 EWRONMEN HE/11TH PERMIT/SERVICES P 0 Box 2165 Livermore 6allfornia 94551 ■ 510-373-9211 OPERATIONAL DEFINITION STEP DRAWDOWN TESTS The step drawdown test is recognized as the most definitive well performance test for use an most aquifers. Step drawdown tests are used to determine well yield, pump setting requirements, and the well efficiency at various pumping rates . A preliminary estimate of the aquifer transmissivity can also be determined from step drawdown tests . A step drawdown test is performed by operating a well pump (or preferably an overrated test pump) during successive time periods at constant fractions of the well 's full capacity. The test pump capacity can be overrated with respect to the well 's capability , or the owner's requirements, if less than the well 's capability. If the pump is of a capacity lower than the well's full capability, careful judgement should be exercised an extrapolating beyond the test limits . . The well is pumped at a low constant discharge rate until the drawdown within the well stabilizes . The pumping rate as then increased as quickly as possible to a higher constant discharge rate until the drawdown in the well once more stabilizes. This process is repeated through at least three steps , which should all be of hydraulically equivalent duration an terms of the stabilization of the drawdown of the water level . The usual duration of each step is from 30 minutes to 2 hours. Step drawdown tests, like all well tests, require accurate measurement of water level in the well at predetermined tames. The usual practice an well testing as that water levels be measured on an expanding time scale because water level response is a function of the logarithm of time. The frequency of water level measurements during a step drawdown test as greater than for a longer term test because for each step the sequence of the time interval of water level measurements as repeated. In some types of aquifers (particularly fractured rock) useful information can be derived from a step drawdown test of water levels are measured very frequently, rather than on an expanding time scale. The vertical distance between the static ( fully recovered, non- pumping) water level and the pump intake (or some agreed height above the intake) as referred to as the maximum available drawdown. In areas of variable static water level caused by seasonal , or other, fluctuations, this effect must be accounted for in selecting pump . settings and pumping rates .